1. Field of the Invention
The present invention relates to a container used in heating by a microwave oven which allows a material to be uniformly and effectively heated, while minimizing any sudden local boiling which may occur during the microwave heating in the microwave oven.
2. Description of the Prior Art
Microwave ovens are widely used in commercial and industrial fields as well as in homes for various purposes including heating and cooking foods, since, with the microwave oven, a container having a lower heat resistance can be used than those required for heating by a gas range or oven, the time required for heating is very short, the contents are not scorched, and operation is simple. Various types of instant foods have, therefore, been marketed which are capable of being reheated or cooked by a microwave oven.
When heating foods by a microwave oven, foods are placed in a metallic cavity within the oven, and are irradiated by microwaves applied from a wave outlet provided on the wall of the cavity. The generated microwaves are reflected by the metal on the wall of the cavity, and penetrate the food from all directions, thereby heating the food uniformly.
In this regard, practical heating by a microwave oven presents the following phenomena in which a food 2 packed in a container body 1 shaped as, for example, shown in FIG. 8 is heated by a microwave oven.
More specifically, a portion 3 at which the container 1 and the upper surface (liquid surface) of the food 2 make contact is irradiated by both microwaves a and b supplied from the roof and the sides of the oven, respectively, while the other portions of the food within the container 1 receive only the microwaves (a or b) supplied from either the roof or from the side of the oven. This causes the portion 3 where the container body 1 and the food 2 make contact to be excessively heated, generating boiling thereon. This at the same time strikingly delays heating of the other portions within the container body 1, particularly the lower portion of the container, thereby causing the food to be heated unevenly.
It has therefore been the practice to provide an aluminum foil covering on the external surface of the container over the portion where a food is excessively heated, thereby overcoming the above described problem. In this improvement, since the aluminum foil reflects microwaves when they hit the foil, the transmission of microwaves is prevented at this portion, causing the food of this portion to be heated slowly. Various other improvements utilizing this prior art have been considered. Japanese Utility Model Publication No. 2367/1976, for example, discloses a container for a Japanese dish known as "chawanmushi" in which a metallic film is provided either on the external surface or on the internal surface of a lid such as to reflect microwaves, and in which a resistant film is also provided on the bottom of the container for absorbing microwaves. Japanese Utility Model Public Disclosure No. 189795/1983 and Japanese Patent Public Disclosure No. 32261/1981 propose a food package having a microwave shielding member provided on the side surface of a package. The specification of Japanese Patent Public Disclosure No. 126743/1979 discloses coating the periphery of food with a metallic foil selected in accordance with the speed at which a food is to be heated.
In all of the above described methods, however, a large portion of a container or its contents is coated with a metal such as an aluminum foil or the like by bonding. Consequently, microwaves are shielded to a greater extent than is desirable, thereby producing large energy loss and delaying the process of heating the contents to a remarkable extent. Also, this often causes the portion of a food which is coated with a metal to remain unheated, thereby often creating uneven heating of the food. Further, the temperature of the edge of the aluminum foil used is abnormally raised due to induction heating, and the portion of a container which contacts this portion is thereby melted or burned.